So far it has been very difficult to determine, with a high degree of accuracy, the three-dimensional shape of a cylindrical, elongated product bent into such a shape and therefore having different straight portions, i.e., an object having portions whose axes cannot be directly determined, because there are many problems with the conventional three-dimensional shape measuring apparatus, i.e., those with measuring devices to brought into contact with the objects. One of the conventional apparatus is disclosed in U.S. Pat. No. 3,944,798 entitled "Method and Apparatus for Measuring Directions". This apparatus determines the three-dimensional shape of the object by finding the vectors of the straight portions of the object. The major problems with the conventional apparatus, including that of the foregoing U.S. patent, are as follows:
1. The measuring device of the conventional apparatus is relatively large in size. Therefore, it cannot be used for measurement of a short straight portion of the object.
2. A measuring device reduced in size may result in a greater measurement error.
3. When a measuring device having a circular end to be brought into contact with the object is used, the operation of the measuring apparatus takes much time and trouble.
4. Contact of the measuring device with the object may cause the object to be moved or deformed during measurement.